1. Field of the Invention
The invention is an aqueous contact adhesive based on EVA copolymers.
Contact adhesives are adhesives which are applied to both substrates to be bonded and, after a dried adhesive film has formed, the substrates are fitted together under pressure and a high strength bond is immediately formed.
Contact adhesives generally comprise synthetic rubbers, such as 2-chlorobutadiene polymers, and resins which are dissolved in organic solvents. In addition, they can contain other additives, such as stabilizers, viscosity regulators and the like. The occasionally high content of solvents and diluents (70 to 80%), which in some cases are toxic and/or readily inflammable, has prompted attempts to develop aqueous contact adhesives.
2. Statement of Related Art
Contact adhesives based on aqueous chlorobutadiene latices (chloroprene latices) do not produce the bond strengths obtainable with chloroprene-solvent contact adhesives. Acrylate copolymers used experimentally, in the form of aqueous dispersions, for contact adhesives show more of the character of pressure-sensitive adhesives and the phenomenon of "cold flow." The effect of cold flow, for example, permits a resistant covering of needle felt bonded around the nose of a projecting stair edge to gradually straighten.
Ethylene/vinyl acetate-based aqueous dispersions are known, having been proposed as a raw material for aqueous contact adhesives, for example in DE-OS No. 32 44 755. Although contact adhesives can be produced with these starting materials, they show only minimal bond strength or minimal adhesiveness for the materials to be bonded.
Water-borne contact adhesives hitherto formulated from commercially available raw materials show either inadequate adhesiveness or cohesion. What obviously matters is that, at the moment of bonding, i.e. when the materials coated with the adhesive are fitted together, the aqueous contact adhesive shows adhesiveness which diminishes after the bonding process and is supplanted by higher cohesiveness. The change from adhesive to cohesive phase must take place as quickly as in solvent-based contact adhesives.